This invention relates to a method of heat treating cold reduced, aluminum-killed low carbon steel strip, and more particularly to a pretreatment of the cold reduced strip within a critical temperature range prior to a continous anneal. By variation in the pretreatment conditions and in composition of the steel, the pretreatment step of this invention results either in attainment of high r.sub.m value or a high yield strength.
An article by R. H. Goodenow, Transactions Of The ASM, Vol. 59, pages 804-823, 1966, discusses the effect of aluminum nitride precipitates on recrystallization and grain structure of low carbon steels. Test data show that an increase in the aluminum nitride content inhibits recrystallization, and that cold working increases the rate of precipitation of aluminum nitride and inhibits the recrystallization process. Isothermal recrystallization curves are presented (p. 806), showing that annealing above 1100.degree. F. required somewhat longer time to recrystallize completely in the case of an aluminum-killed steel containing 0.041% acid soluble aluminum and 0.0007% nitrogen as aluminum nitride. At 1050.degree. F. the recrystallization curve departed from "the normal sigmoidal-shaped curve". At 1000.degree. F. recrystallization stopped after 83% had recrystallized in a time interval up to 48 hours. Below 975.degree. F. no recrystallization occurred up to 48 hours. The inhibiting effect of aluminum nitride on recrystallization in aluminum-killed steel was examined in a two-stage isothermal anneal comprising a heat treatment below 975.degree. F. followed by heating up to 1300.degree. F. Recrystallization occurred at 1300.degree. F., but the time to start and completion of recrystallization increased with increasing aluminum nitride content. Similar results were obtained with initial heat treatments at 900.degree. and 850.degree. F. It was further observed that low temperature heat treatment, which promotes aluminum nitride precipitation in a time-dependent manner, causes a change in the recrystallized texture, mainly " . . . an increase in the relative intensity of the (111) component" (page 821).
No application of the findings of the article is suggested which would indicate possible benefits in obtaining higher strength, or better drawability with a continuous anneal.
Japanese No. 57073-125, published May 7, 1982, discloses subjecting a steel containing up to 0.15% carbon, up to 0.6% silicon, 0.5% to 1.6% manganese, 0.01% to 0.10% acid soluble aluminum, 0.04% to 0.80% chromium, 0.0005% to 0.003% boron and/or 0.02% to 0.4% vanadium, and balance iron, to hot rolling, coiling at 200.degree. to 620.degree. C., cold reducing by at least 40%, box annealing at 400.degree. C. to the A.sub.1 point, and then continuously hot dip zinc coating. It is alleged that the manganese content produces "improved delayed ageing property and paint-baking hardenability". The box annealing step before zinc coating is stated to enhance the r.sub.m value.
The effect of columbium and/or zirconium in retarding recrystallization is disclosed in U.S. Pat. Nos. 3,761,324; 3,963,531 and 4,067,754. No. 4,067,754 discloses annealing of a cold reduced low carbon strip at a temperature of about 1100.degree. to 1300.degree. F. for about 7 minutes to 24 hours, with the time inversely proportional to the temperature, whereby to recover ductility but not recrystallize and to achieve a yield strength of at least 90 ksi and an elongation of greater than 10%. If batch annealed at 1200.degree. to 1400.degree. F. (with a minimum of 4 hours at 1200.degree. F.) or continuously annealed at 1500.degree. to 1700.degree. F. a fully recrystallized structure is obtained having a yield strength of 45 to 65 ksi and an elongation greater than 25%.